Carrier Head Membrane Roughness to Control Polishing Rate

ABSTRACT

An apparatus comprises a flexible membrane for use with a carrier head of a substrate chemical mechanical polishing apparatus. The membrane comprises an outer surface providing a substrate receiving surface, wherein the outer surface has a central portion and an edge portion surrounding the central portion, wherein the central portion has a first surface roughness and the edge portion has a second surface roughness, the first surface roughness being greater than the second surface roughness.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 61/122,322, filed on Dec. 12, 2008, the entire contents of which isincorporated by reference.

BACKGROUND

This invention relates to chemical mechanical polishing, and moreparticularly to a flexible membrane for use a carrier head for chemicalmechanical polishing.

Integrated circuits are typically formed on substrates, particularlysilicon wafers, by the sequential deposition of conductive,semiconductive or insulative layers. After each layer is deposited, itis etched to create circuitry features. As a series of layers aresequentially deposited and etched, the outer or uppermost surface of thesubstrate, i.e., the exposed surface of the substrate, becomesincreasingly nonplanar. This nonplanar surface can present problems inthe photolithographic steps of the integrated circuit fabricationprocess. Therefore, there is a need to periodically planarize thesubstrate surface. In addition, planarization is needed when polishingback a filler layer, e.g., when filling trenches in a dielectric layerwith metal.

Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier or polishing head. The exposed surfaceof the substrate is placed against a polishing pad, such as circular pador linear belt, that moves relative to the substrate. The carrier headprovides a controllable load on the substrate to push it against thepolishing pad. Some carrier heads include a flexible membrane thatprovides a mounting surface for the substrate, and a retaining ring tohold the substrate beneath the mounting surface. Pressurization orevacuation of a chamber behind the flexible membrane controls the loadon the substrate. A polishing liquid, such as a slurry with abrasiveparticles, is supplied to the surface of the polishing pad.

A reoccurring problem in CMP is non-uniform polishing, i.e., variationin the polishing rate across the substrate surface. For example,polishing may thoroughly remove conductive material in some areas whileleaving conductive material residue in other areas.

SUMMARY

In one aspect, a membrane is described for use with a carrier head of asubstrate chemical mechanical polishing apparatus. The membrane has anouter surface providing a substrate receiving surface. The outer surfacehas a central portion and an edge portion surrounding the centralportion. The central portion has a first surface roughness and the edgeportion has a second surface roughness, the first surface roughnessbeing greater than the second surface roughness.

Embodiments of the membrane may include one or more of the followingfeatures. The edge portion of the outer surface of the membrane furthercomprises a peripheral portion extending upward from the outer surface.The second surface roughness is less than about 5 microinches. The firstsurface roughness is greater than about 15 microinches. A width of theedge portion is less than about 25 percent of a width of the centralportion. The membrane further comprises features that have a first meanspacing over the central portion and a second mean spacing over the edgeportion. The features in the central portion are roughly sphericalbumps, and the features in the edge portion are of negligible size. Themembrane comprises at least one material from a group consisting ofchloroprene, silicon, and ethylene propylene diene M-class rubber, andthe features of the membrane consist of the same material as themembrane. The edge portion and the central portion consist of the samematerial.

The membranes described herein can be used with a carrier head with aretaining ring. The membrane includes an inner surface that forms aboundary of a pressurizable chamber. The retaining ring surrounds theflexible membrane.

A method of using the membranes described herein is provided. The methodincludes applying a load to a substrate with a carrier head. A relativemotion is created between the substrate and a polishing pad whileapplying the load. The relative motion causes polishing of the substrateand the second surface roughness causes an edge of the substrate to bepolished at a rate greater than polishing the substrate with a carrierhead having a membrane having the first roughness across an entirety ofthe outer surface. The substrate is released from the membrane. Thecentral portion of the flexible membrane enables release of thesubstrate from the membrane.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view of a carrier head.

FIG. 2 is an illustration of a flexible membrane for use in a carrierhead.

FIG. 3 is a schematic cross-sectional view of part of a flexiblemembrane for use in a carrier head.

FIG. 4 is an illustration of a die map across a substrate.

FIG. 5 is a schematic bottom view of an outer surface of a flexiblemembrane. Like reference symbols in the various drawings indicate likeelements.

DETAILED DESCRIPTION

One reoccurring problem in CMP is underpolishing at an edge portion ofthe substrate. While various solutions, such as applying additional loadto an edge of the substrate during polishing, can equalize the polishingrate across the substrate, the solutions often require redesign of anon-consumable part used in polishing, that is, the carrier head. Amembrane used to back the substrate during polishing that has regions ofdecreased roughness can control the polishing rate in regions of thesubstrate that otherwise tend to be underpolished. A membrane with asubstrate backing surface that has a smooth region surrounding a roughregions can provide uniform polishing across a substrate withoutmodifying the carrier head itself.

Referring to FIG. 1, one or more substrates 10 will be polished by achemical mechanical polishing (CMP) apparatus that includes a carrierhead 100. A description of a similar CMP apparatus may be found in U.S.Pat. No. 5,738,574, the entire disclosure of which is herebyincorporated by reference.

Referring to FIG. 1, an exemplary carrier head 100 includes a housing102, a base 104, a gimbal mechanism 106, a loading chamber 200, aretaining ring 110, and a flexible membrane 118 with a substratereceiving surface that has regions of differing surface roughness. Adescription of a carrier head that can be used with such a membrane maybe found in U.S. Patent Publication No. 2008-0119118, U.S. Pat. No.6,857,945, U.S. Pat. No. 6,422,927, U.S. Pat. No. 6,277,014, U.S. Pat.No. 6,183,354 or U.S. Pat. No. 6,146,259, the entire disclosures ofwhich are hereby incorporated by reference. Other types of carrier headswith different components can alternatively be used and carrier head 100is described merely as an example of one type of carrier head. Flexiblemembrane 118 extends below base 104 to provide a mounting surface 274for the substrate. Pressurization of a chamber 276 positioned below base104 forces flexible membrane 118 downwardly to press the substrateagainst the polishing pad. Flexible membrane 118 is further illustratedin FIG. 2. Flexible membrane 118 is a generally circular sheet formed ofa flexible and elastic material, such as a polymer of chloroprene,ethylene propylene, silicon, ethylene propylene diene M-class rubber orsilicone rubber. A perimeter portion of flexible membrane 118 is securedto the base, e.g., clamped between the retaining ring 110 and the base104. A bead 254 is formed around the boundary, e.g., outer edge of theperimeter portion of flexible membrane 118. In some embodiments, one ormore cylindrical flaps extend upward from a main portion 208 of themembrane and bead 254 is at the end of cylindrical flap 252.

FIG. 3 illustrates a diametric cross-section of part of flexiblemembrane 118. Referring back to FIG. 1, a bottom surface or the mountingsurface 274 of flexible membrane 118 is in contact with substrate 10during polishing. The sealed volume between flexible membrane 118, andbase 104 defines pressurizable chamber 276. A pump (not shown) may befluidly connected to chamber 276 to control the pressure in the chamberand thus the downward forces of the flexible membrane on the substrate.In operation, fluid is pumped into chamber 276 to control the downwardpressure applied by flexible membrane 118 against the center portion ofthe substrate. When chamber 276 is pressurized, flexible membrane 118will also expand laterally outward.

As noted above, a reoccurring problem in CMP is underpolishing at anedge portion of the substrate. Consequences of such underpolishing areillustrated in FIG. 4. Each square in the diagram represents a die on awafer substrate. The edge dies 343 with “X”s on an edge portion of thesubstrate represent portions with conductive material residue, such ascopper residue, after polishing has completed at a central region of thesubstrate. If the substrate were polished sufficiently long to removeall of the residue from edge dies 343, overpolishing can occur atcentral dies 360. Overpolishing can result in dies with dishing orinsufficient conductive material where conductive features are required.Polishing completion means that a target layer, e.g., copper, has beenremoved from over certain regions, such as non-conductive regions andbetween features on a die.

Polishing rates can be affected by a surface roughness of an outersurface of flexible membrane 118. In some embodiments, a surfaceroughness less than about 5 microinches has a higher polishing rate thanthe membranes with higher surface roughness values. A membrane withareas of varying surface roughness can be used to address the polishingrequirements at different areas on the wafer.

Referring to FIG. 5, in order to address underpolishing, such as at anedge portion of a substrate, the outer surface of flexible membrane 118has a central portion 422 and an edge portion 424 surrounding centralportion 422 with different surface roughness values. In someembodiments, the central portion of the outer surface of flexiblemembrane 118 has a surface roughness than the edge portion of the outersurface of flexible membrane 118. In some embodiments, the centralportion's surface roughness is greater than about 15 microinches, suchas greater than 20 microinches. In some embodiments, the edge portion'ssurface roughness is less than about 10 microinches, such as less thanabout 5 microinches, such as less than 3 microinches. The surfaceroughness of the edge portion 424 is less than the surface roughness ofthe central portion and in some embodiments, the surface roughness ofthe central portion is sufficient to prevent the membrane from stickingto a backside of a substrate. The precise surface roughness required toprevent sticking to the substrate can depend on the membrane material.Also, if the membrane is used in combination with a carrier head that isable to provide different loads to different parts of a substrate duringpolishing, the differential in roughness in the central portion 422 andthe edge portion 424 may not need to be as great. For example, thecentral portion 422 can have a surface roughness greater than 10microinches and the edge portion can have a surface roughness less than10 microinches. Alternatively, the central portion 422 can have asurface roughness greater than 5 microinches and the edge portion canhave a surface roughness less than 5 microinches. the central portion422 can have a surface roughness greater than 15 microinches and theedge portion can have a surface roughness less than 15 microinches.

The edge portion of the outer surface of flexible membrane 118 with thereduced surface roughness can have a width that is less than about 25percent of a width of the central portion of the outer surface offlexible membrane 118, such as less than 20%, less than 15%, less than10% or less than 5% of the width of the central portion. The ratio ofthe width of the central portion to the ratio of the width of the edgeportion can depend on the region of the substrate that needs increasedpolishing at the edge. In some embodiments, at least 50% of the membranesurface has a roughness greater than 15 microinches to reduce a surfacetension between the membrane and the substrate to dechucking, asdescribed below. In some embodiments, the central portion 422 and theedge portion 424 are composed of the same material. That is, there is nocoating of a different material from the membrane required to form thesmooth or rough regions on the membrane. In some embodiments, flexiblemembrane 118 has a surface roughness profile across the membrane suchthat the gradient of the surface roughness profile with respect to adistance from a center point of the membrane is negative. In someembodiments, the central portion has a circular shape and the edgeportion has an annular shape.

Surface roughness profiles can be generated via a mold designed to formfeatures of varying size and density on the outer surface of flexiblemembrane 118. In this way, the features can be formed of the samematerial as that of flexible membrane 118. The mold can be formed of arigid material, such as stainless steel. A portion of the mold can bepolished to form the smooth region or regions of the membrane and aportion of the mold can be treated, such as by sandblasting, to form therougher regions of the membrane. In some embodiments, a mold can beconstructed such that a flexible membrane formed by the mold hasfeatures that have a first mean spacing over the central portion of theouter surface of the flexible membrane and a second mean spacing overthe edge portion of the outer surface of the flexible membrane. In someembodiments, the features in the central portion 422 may take the formof e.g., spherical bumps, pyramidal bumps, or linear bumps, and thefeatures in the edge portion 424 are of negligible size or the edgeportion 424 lacks bumps.

Polishing a substrate with carrier head using a flexible membrane havinga smooth edge region and a rougher central region involves applying aload to substrate with the carrier head by increasing a pressure withina pressurizable chamber defined in part by the membrane. As statedabove, the increase of pressure can be effected by introducing a fluidinto pressurizable chamber.

The carrier head then creates a relative motion between substrate andpolishing pad while applying the load. While the relative motion causespolishing of the substrate, the difference in surface roughness betweenthe central and edge portions of the outer surface of flexible membranecauses a difference in polishing rates in the corresponding regions ofsubstrate. The greater surface roughness at the edge of the membranecauses an edge of the substrate to be polished at a rate greater thanpolishing the substrate with a carrier head using a membrane having thea uniform roughness across an entirety of the outer surface. Inpolishing processes which tend to cause underpolishing in the edgeregions of the substrate with a uniform surface roughness profile, sucha multizone rough and smooth membrane may result in uniform polishingacross substrate.

Upon completion of the polishing of substrate, the carrier head ideallyreleases substrate without breaking the substrate. Typically, release ofsubstrate is achieved through pressurizing the pressurizable chamberbacking the substrate until at least a portion of substrate is separatedfrom the flexible membrane. An increased surface roughness in thecentral portion of the outer surface of flexible membrane results in aless sticky surface. The less sticky surface enables separation of thesubstrate from flexible membrane and eases dechuck.

The apparatuses and methods described above have been described in termsof a number of embodiments. The apparatuses and methods, however, arenot limited to the embodiments depicted and described. Accordingly,other embodiments are within the scope of the following claims.

1. An apparatus, comprising: a flexible membrane for use with a carrierhead of a substrate chemical mechanical polishing apparatus, themembrane comprising: an outer surface providing a substrate receivingsurface, wherein the outer surface has a central portion and an edgeportion surrounding the central portion, wherein the central portion hasa first surface roughness and the edge portion has a second surfaceroughness, the first surface roughness being greater than the secondsurface roughness.
 2. The apparatus of claim 1, wherein the edge portionof the outer surface of the membrane further comprises a peripheralportion extending upward from the outer surface.
 3. The apparatus ofclaim 1, wherein the second surface roughness is less than about 5microinches.
 4. The apparatus of claim 1, wherein the first surfaceroughness is greater than about 15 microinches.
 5. The apparatus ofclaim 1, wherein a width of the edge portion is less than about 25percent of a width of the central portion.
 6. The apparatus of claim 1,wherein the membrane further comprises features that have a first meanspacing over the central portion and a second mean spacing over the edgeportion.
 7. The apparatus of claim 6, wherein the features in thecentral portion are roughly spherical bumps, and the features in theedge portion are of negligible size.
 8. The apparatus of claim 6,wherein the membrane comprises at least one material from a groupconsisting of chloroprene, silicon, and ethylene propylene diene M-classrubber, and the features of the membrane consist of the same material asthe membrane.
 9. The apparatus of claim 8, wherein the edge portion andthe central portion consist of the same material.
 10. An apparatus,comprising: a carrier head comprising: a retaining ring; and a flexiblemembrane configured to press a substrate against a polishing pad, themembrane including an inner surface that forms a boundary of apressurizable chamber and an outer surface providing a substratereceiving surface; wherein the retaining ring surrounds the flexiblemembrane and the outer surface comprises a central portion and an edgeportion surrounding the central portion, wherein the central portion hasa first surface roughness and the edge portion has a second surfaceroughness, the first surface roughness being greater than the secondsurface roughness.
 11. The apparatus of claim 10, wherein the secondsurface roughness is less than about 5 microinches.
 12. The apparatus ofclaim 10, wherein the first surface roughness is greater than about 15microinches.
 13. The apparatus of claim 10, wherein an edge of themembrane is attached to the retaining ring so that a fluid-tight sealforms around the pressurizable chamber.
 14. The apparatus of claim 10,wherein the membrane further comprises features that have a first meanspacing over the central portion and a second mean spacing over the edgeportion.
 15. The apparatus of claim 14, features in the central portionare roughly spherical bumps, and the edge portion is substantially freeof spherical bumps.
 16. The apparatus of claim 14, wherein the membranecomprises at least one material from a group consisting of chloroprene,silicon, and ethylene propylene diene M-class rubber, and the featuresof the membrane consist of the same material as the membrane.
 17. Amethod comprising: applying a load to a substrate with a carrier head,the carrier head comprising: a retaining ring; and a flexible membraneconfigured to press the substrate against a polishing pad, the membraneincluding an inner surface that forms the boundary of a pressurizablechamber and an outer surface providing a substrate receiving surface;wherein the retaining ring surrounds the flexible membrane and the outersurface comprises a central portion and an edge portion surrounding thecentral portion, wherein the central portion has a first surfaceroughness and the edge portion has a second surface roughness, the firstsurface roughness being greater than the second surface roughness;creating a relative motion between the substrate and a polishing padwhile applying the load, wherein the relative motion causes polishing ofthe substrate and the second surface roughness causes an edge of thesubstrate to be polished at a rate greater than polishing the substratewith a carrier head having a membrane having the first roughness acrossan entirety of the outer surface; and releasing the substrate from themembrane, wherein the central portion of the flexible membrane enablesrelease of the substrate from the membrane.
 18. The method of claim 17,wherein the membrane comprises: at least one material from a groupcomprising: chloroprene, silicon, and ethylene propylene diene M-classrubber; and features that have a first mean spacing over the centralportion and a second mean spacing over the edge portion; wherein thefeatures of the membrane consist of the same material as the membrane.19. The method of claim 17, wherein polishing comprises increasing apressure within the pressurizable chamber against the inner surface ofthe membrane.
 20. The method of claim 17, wherein releasing comprisesdecreasing a pressure within the pressurizable chamber against the innersurface of the membrane until at least a portion of the membrane isseparated from the substrate.